As worldwide environmental regulations are becoming more strict, people place a higher requirement upon quality of gasoline products. For instance, China has implemented State IV discharge criteria for oil products nationwide since Jan. 1, 2014, which requires sulfur content of gasoline to be reduced below 50 ppm; meanwhile, China has also put forward State V discharge criteria, which requires sulfur content to be reduced below 10 ppm and olefin content to be controlled below 24%.
Compared with developed countries, China's gasoline has a relatively high content of sulfur, which is mainly due to the fact that about 70-80% of China's gasoline comes out from a fluid catalytic cracking (FCC) process. In the gasoline products, about 90% of olefin content and sulfur content comes out from the fluid catalytic cracking gasoline, which causes that China's gasoline products are far from meeting a requirement for new criteria where sulfur content ≦10 ppm and olefin content ≦24%. Thus, reduction of sulfur content in the fluid catalytic cracking gasoline is a key to upgrading quality of China's motor gasoline.
Hydrodesulfurization is the most effective method to remove sulfide from gasoline. Sinopec Research Institute of Petroleum Processing developed an FCC gasoline selective hydrodesulfurization process (RSDS-I) in 2001, where FCC gasoline is firstly cut into a light fraction and a heavy fraction at a cutting temperature of 90° C., and then the light fraction is subjected to alkali extraction mercaptan removal, and the heavy fraction is subjected to selective hydrodesulfurization using a main catalyst of RSDS-I and a protective agent of RGO-2; and in a second generation of FCC gasoline selective hydrodesulfurization technique (RSDS-II) for improvements on the process above, a cutting point of the light fraction and heavy fraction is decreased to 70° C., and a second generation of hydrogenation catalysts RSDS-21 and RSDS-22 are used in a selective hydrodesulfurization portion of the heavy fraction.
Axens Corporate of French Institute of Petroleum (IFP) developed a Prime-G+ process, where a process flow of full range pre-hydrogenation, light and heavy gasoline cutting and heavy fractions selective hydrodesulfurization is used, and the cutting temperature is set between 93-149° C. according to a target value of sulfur content, and during the full range pre-hydrogenation process, light sulfide reacts with diolefin in the presence of a catalyst of HR845 to form sulfide with a high boiling point, thus olefin is not saturated; furthermore, two catalysts of HR806 and HR841 are used in the selective hydrodesulfurization of the heavy fraction, thus the operation is more flexible.
Sinopec Fushun Research Institute of Petroleum and Petrochemicals developed an OCT-M process, where FCC gasoline is cut into light and heavy fractions at a cutting temperature of 90° C., in which the light fraction is subjected to demercaptan and the heavy fraction is subjected to selective hydrodesulfurization using a combined catalyst of FGH-20/FGH-11.
Hai shunde Special Oil Co., Ltd developed an HDDO series diolefin removal catalyst, an HDOS series deep hydrodesulfurization catalyst, an HDMS series mercaptan removal catalyst and a corresponding FCC gasoline selective hydrodesulfurization process (CDOS), where FCC gasoline is subjected to diolefin removal reaction at a relatively low temperature in a hydrogen condition, then the FCC gasoline is cut into light and heavy components, the heavy fraction is subjected to deep hydrodesulfurization, and the hydrogenated heavy fraction is reconciled with the light fraction to obtain a clean gasoline with less sulfur.
The above techniques have a common problem that the light fraction formed by the cutting has a low yield, and there are fewer components having a content less than 10 ppm, and it is difficult to reduce sulfur content of the light fraction below 10 ppm by means of mercaptan removal only; when gasoline products having sulfur content less than 10 ppm are produced, a majority of light fraction still need to be hydrodesulfurized, thus loss of octane number of full range gasoline is higher (for instance, up to 3.0-4.0). Furthermore, even though the sulfur content is allowed to be less than 10 ppm by means of hydrodesulfurization, there are still defects that investment and operational costs are high, and a large number of olefin is saturated while sulfide is removed, which not only increases hydrogen consumption, but also reduces octane number of gasoline greatly.
The adsorption desulfurization may be carried out at a room temperature and atmospheric pressure with low energy consumption and almost no loss of octane number, which is a relatively potential method for deep desulfurization, and which is reported mostly at present. For instance, an IRVAD technique jointly developed by Black & Veatch Pritchard Inc. and Alcoa Industrial Chemicals employs multi-stage fluidized bed adsorption method, which uses an aluminum oxide substrate as a selective solid adsorbent to process liquid hydrocarbons, during the adsorption, the adsorbent is countercurrent in contact with the liquid hydrocarbons, the used adsorbent countercurrently reacts with a regenerated thermal current (such as hydrogen) for regeneration. A desulfurization rate of this technique can reach above 90%, however, this adsorbent is of less selectivity, sulfur adsorption capacity thereof is limited, and the regeneration process is relatively complex.
Philips Petroleum Company developed an S-Zorb process where a specific adsorbent is used for desulfurization in a hydrogen condition, the adsorbent takes zinc oxide, silicon dioxide and aluminium oxide as a carrier and loads metal components such as Co, Ni, Cu, etc., which can absorb a sulfur atom in sulfide to maintain it on the adsorbent, whereas the hydrocarbon structure part of the sulfide is released back to the process stream so as to implement a desulfurization process. This process does not generate H2S during the reaction, thereby preventing H2S from reacting with olefin again to generate mercaptan. However, the desulfurization technique places a relatively harsh requirement upon process operation conditions, the temperature of the desulfurization reaction is 343-413 and the pressure is 2.5-2.9 MPa.
The adsorbent of desulfurization described above cannot be better used in the selective hydrodesulfurization of the heavy fraction due to problems such as limited deep desulfurization and small sulfur adsorption capacity, low selectivity, short lifespan, relatively complex regeneration process and harsh desulfurization conditions. Thus, there is a pressing demand to develop a method for deep desulfurization of gasoline, of which loss of octane number is less, desulfurization degree is highly deep, and the operation is convenient and flexible.